Dark matter critics focus on details, ignore big picture

Theorists behind gravity alternatives polishing bumper of a car with no wheels.

Way back in the olden days, when the world was young and we were all much more innocent*, the Bullet Cluster was discovered. It was a revelation: two galaxy clusters had collided, leaving a mess of glowing ordinary matter and large clumps of invisible, gravity-rich material—in completely separate locations. Combined with some modeling, this was hailed as an unambiguous observation of dark matter.

Scientists, who were thinking of committing the sacrilege** of modifying the theory of gravity in a way that made dark matter superfluous, were saved. Our existing theory of gravity seemed to work, and they could return to reveling in the certainty of the known and simply reap the reward of... well, certainly there was some sort of reward.

Unfortunately, someone forgot to put the lens cap back on the Hubble telescope. Much to everyone's dismay, it snapped a picture of the Abell 520 cluster, which had dark matter where it shouldn't be. To make matters worse, it certainly didn't have visible matter where it should. Those few scientists who, disturbingly, enjoy thinking dark and sacrilegious thoughts were back in action. They might be allowed to modify gravity after all.

So all eyes turned to Israel, where Mordehi Milgrom pondered his differential equations and said, "Well, if we're going to modify gravity, we should make sure that it lines up with observations (at least some of them, anyway)." He has labored mightily to see if modified Newtonian dynamics (MOND) can predict the rotation curves of more galaxies.

Originally, MOND was an alternative explanation for observations of the speed with which stars move in certain galaxies, called rotation curves. At the time, either explanation—modified gravity or dark matter—were equally valid.

But over the years, cosmology entered an age of precision. We got details from the cosmic microwave background that told us a lot about the Universe's structure and evolution. And suddenly we could calculate how the Universe would look like without dark matter. It didn't look much like the one we observe today. This new line of evidence, was, I think, the factor that has driven many physicists to accept dark matter as a reality.

But MOND is not dead, and rightly so, because, the two observations that I just described (the Bullet and Abell 520 clusters) show that our understanding of dark matter is far from complete***. But MOND theories, which are supposed to do away with the need for dark matter, have been in a similar position—they can't explain both of these observations, either.

Milgrom's latest paper shows that MOND theories can account for the rotation curves (the speed of stars within a galaxy) of a pair of elliptical galaxies. The same MOND models also nicely account for the rotation curves of spiral and other galaxy types. This is useful, since it shows that a consistent MOND framework can account for a range of different observations. Coupled with the fact that MOND can, reportedly, account for the Bullet cluster, the case for MOND begins to look stronger.

The other ray of hope that is opening up for MOND theorists are the results from the Large Hadron Collider. Physicists have been hoping that the LHC would produce dark matter particles in the debris of its collisions. However, observations so far show no hint of a stable dark matter particle.

Nevertheless, MOND still has to be able to account for the cosmic microwave background and Abell 520 cluster. The cosmic microwave background is especially problematic; if that cannot be explained, MOND cannot pass muster.

This is what puzzles me most. I know it's important to check the sort of details like galaxy structures, but I really want to know if they are making progress on the bigger problems. Put simply, I would rather have a theory that explained the big picture and messed up some details, rather than a theory that got a particular set of details completely right and failed to describe the big picture.

But this is science in action. Both theories have some evidence going for them. Both have troubling observations. And, importantly, even though they take potshots at each other over dinner at conferences, neither side is excluded from the conversation, as long as they both work to accommodate the data from the real world.

160 Reader Comments

To go out on a limb a bit, there will be no such thing Dark Matter; what they will find, is that normal matter can "over displace" space-time, and manifest more gravity than it should.

I think it will be found that this phenomena occurs in nature primarily in black holes. Such over-pumped gravity may very well manifest over a larger area than it should, in addition to simply being stronger than the given mass should produce.

And yeah, that would mean that the gravitational constant is not so constant after all. I don't have to tell you smart people where that leads. I just hope this dark matter fetish doesn't distract science for too long a time.

gravity doesn't 'travel' as you say here, it's effect is felt because space time is curved by the presence of a large mass. Plenty of photons are emitted by a black hole (look up Hawking Radiation if you don't believe me.

That's what you say. It's not what the people trying to build gravity wave detectors believe.

To go out on a limb a bit, there will be no such thing Dark Matter; what they will find, is that normal matter can "over displace" space-time, and manifest more gravity than it should.

I think it will be found that this phenomena occurs in nature primarily in black holes. Such over-pumped gravity may very well manifest over a larger area than it should, in addition to simply being stronger than the given mass should produce.

And yeah, that would mean that the gravitational constant is not so constant after all. I don't have to tell you smart people where that leads. I just hope this dark matter fetish doesn't distract science for too long a time.

Or in other words, spacetime is lumpy. Some places have more "there" in them. And that looks like gravity.

Gravitational waves are a prediction of general relativity arising from a quadrupole moment. Nothing about gravitational waves is inconsistent with Einstein's interpretation of gravity as curvature of spacetime. Indeed, finding no gravitational waves would disprove GR.

To go out on a limb a bit, there will be no such thing Dark Matter; what they will find, is that normal matter can "over displace" space-time, and manifest more gravity than it should.

I think it will be found that this phenomena occurs in nature primarily in black holes. Such over-pumped gravity may very well manifest over a larger area than it should, in addition to simply being stronger than the given mass should produce.

And yeah, that would mean that the gravitational constant is not so constant after all. I don't have to tell you smart people where that leads. I just hope this dark matter fetish doesn't distract science for too long a time.

That is a rather bold statement to make about something that hundreds people have been working on for decades.

As has been previously noted, there are successes for positing "undetectable" phenomena (I know, it's not completely undetectable) that have later been show to be correct. There have also been misses, but the point is that you can't always be sure.

We aren't sure about gravity as a observed phenomenon. There are all sorts of theories about the mechanism behind it, everything from gravitons to dismissing the idea that it is a fundamental force at all. However science has shown a remarkable tendency to move towards the "right" answer.

Asimov's famous essay The Relativity of Wrong shows that just because a theory is imperfect doesn't make it "wrong". Or another way, that being "wrong" isn't necessarily the opposite of being right. In this context at least.

Put me in the 'critical of dark matter' camp. Now, I only have my master degree in astrophysics (received in the university of the 'inventor' of dark matter no less...) and went on to new things after that, so I haven't got the most up-to-date understanding. However, I have never seen any observation that has really convinced me otherwise of my most basic observation of dark matter theory; That it in itself is a band-aid put there to explain observations that do not match our expectations using our current understanding of gravity.

I understand that it fits very well, but it fits very well like string theory fits very well. It is smart, it is tempting, but it is ultimately conceived to fix a hypothesis, not from observation. This is further strengthened by the fact that observations that do not actually prove the existence of dark matter, are used as proof. We already know that the models of gravity do not work without something to adjust, another observation proving this, does nothing to proof that the explanation is dark matter. As far as I know, any theory that produces falsifiable hypotheses has failed thus far or has yet to produce any evidence that supports it.Not unless we get a much firmer grip on what the dark matter might be will I ever be convinced I think.

A lot of the proof is also definitely circular in nature. We can not proof that dark matter has to exist by using gravitational models, since dark matter exists as an explanation for the discrepancy of our understanding of gravity and observations. I understand the tendency to use general relativity as 'fact', since any adjustment is inconceivable to most of us, but as scientists we must not fall into this trap.

Up until we find evidence of massive neutrino's, WIMPS and what not, I think it simply pays to remind ourselves of the 'weakness' of the dark matter theory. It is too easily accepted imho and I am happy at least some people are looking into alternatives.

Short summary: predicted solely because it made the math work, wasn't integrated into theory for years, didn't get detected for decades. There's a history here that those with "i don't like the feel of this" tendencies are largely ignoring.

To be a little blunt, these kind of posts are a bit silly. History has nothing to do with it, especially since we also have examples of the exact opposite happening (epicycles, ether, monopoles?). The fact of the matter is that no one felt 'safe' before we had some useful observations of the whatever was postulated itself. Until such a time we should always remain critical. The lack of that is worrying, since to me it is such a basic scientific principle.

This really is akin to people that say: "well, they also thought the earth was flat" to defend irrational ideas.

I just can't understand that there are many scientists that are truly amazed by how well dark energy and dark matter 'fits', even though that is the sole purpose of why it was 'invented'.

That is a rather bold statement to make about something that hundreds people have been working on for decades.

As has been previously noted, there are successes for positing "undetectable" phenomena (I know, it's not completely undetectable) that have later been show to be correct. There have also been misses, but the point is that you can't always be sure.

We aren't sure about gravity as a observed phenomenon. There are all sorts of theories about the mechanism behind it, everything from gravitons to dismissing the idea that it is a fundamental force at all. However science has shown a remarkable tendency to move towards the "right" answer.

Asimov's famous essay The Relativity of Wrong shows that just because a theory is imperfect doesn't make it "wrong". Or another way, that being "wrong" isn't necessarily the opposite of being right. In this context at least.

Yep. Although I wouldn't say that theorizing the neutrino was quite the same as dark matter theory; I get your point. I've got no skin in the game; if they find gravitons and dark matter in my lifetime, I'll simply be (pleasantly) surprised.

And also yes, you really can't be interested in this stuff and not be a fan. For the few that haven't yet read it:

Gravity is thought to be the result of gravitons, quanta that carry gravity. And they are thought to travel at the speed of light.

No, that's only one hypothesis. Fact is, we don't have a proper understanding of why gravity works.

Agreed. That's why I used the word "thought" in both sentences.

Yes, but it is rude to accuse someone of having a "high school" understanding of something, when the are correctly describing the best supported theory of gravity we have rather than a completely unconfirmed hypothesis.

"Matter tells space how to curve..."

The idea that many people get from high school physics is that space existed before the big bang; it was infinite and flat. Then the big bang and the energy and mass of the universe started to expand into space. Thus, they conclude that matter causes space to curve.

But the most accepted theory in physics is that space is no bigger than the energy and mass it contains. Space was tightly curled up and uncurls as the universe expands. It was never flat and it will never be flat, but it keeps getting flatter as the universe expands.

I blame the lack of quality in the school education system. I didn't mean to insult the person who held these beliefs but the person(s) who gave it to them. I will try to be clearer in the future.

Short summary: predicted solely because it made the math work, wasn't integrated into theory for years, didn't get detected for decades. There's a history here that those with "i don't like the feel of this" tendencies are largely ignoring.

To be a little blunt, these kind of posts are a bit silly. History has nothing to do with it, especially since we also have examples of the exact opposite happening (epicycles, ether, monopoles?).

You misunderstand my argument completely. It wasn't made as a way of saying "neutrinos worked out, therefore dark matter will." It was to point out to all those people saying "this isn't how science works!" that, in fact, it is how science has worked. When you have a result that seems to contradict a theory that works in many, many contexts, your first thought isn't "let's throw out the theory." Your first thoughts are "something's wrong with the data" and "we're missing something" (see, for example, the response to faster than light neutrinos).

This gets into your "fix a hypothesis, not from observation" argument. Dark matter was motivated by data - it was proposed as one possible way of keeping data consistent with a very successful theory (not a hypothesis). Modifying the theory (which is what MOND does) is also being considered. The problem is that we keep getting more data - from the CMB, from gravitational lensing studies, etc. - that's most consistent with the existence of dark matter.

We're now well past the point where it's a hack to keep relativity working. As the article noted, we've built models of the universe with dark matter in them, and they produce a Universe that looks like ours. What it didn't mention is that their output predicted the existence of dark matter filaments between galaxy clusters. We've now used gravitational lensing to find one of these filaments. So, it's nothing at all like string theory, unlike what you claim (and, besides, weren't you also complaining about making false analogies?).

Short summary: predicted solely because it made the math work, wasn't integrated into theory for years, didn't get detected for decades. There's a history here that those with "i don't like the feel of this" tendencies are largely ignoring.

To be a little blunt, these kind of posts are a bit silly. History has nothing to do with it, especially since we also have examples of the exact opposite happening (epicycles, ether, monopoles?).

You misunderstand my argument completely. It wasn't made as a way of saying "neutrinos worked out, therefore dark matter will." It was to point out to all those people saying "this isn't how science works!" that, in fact, it is how science has worked. When you have a result that seems to contradict a theory that works in many, many contexts, your first thought isn't "let's throw out the theory." Your first thoughts are "something's wrong with the data" and "we're missing something" (see, for example, the response to faster than light neutrinos).

I think you missed my point which was, that the fact that this viewpoint worked here and didn't there, says nothing. The fact that we both agree on what is more or less the 'answer' now, is based on very different type of data, that verifies the actual hypotheses based on the observation that something was 'off'. Everything (including the filaments) we have currently, is just an extension of the same 'something is not right here' observation. We just keep verifying that something is indeed not right and that it is exactly not right as we thought it is not right (that was a bit of a weird sentence, but you get my meaning). It doesn't change our understanding of the underlying concept at all and is, imho of course, a waste of time and money.

Also, I'm certainly not stating that MOND is an alternative or dark matter is definitely wrong, not only doesn't it really work, it is also a similar way of approaching the problem, it just picks a different 'fix'.

Doesn't it raise any alarms for you that we have to invent a mysterious dark energy and dark matter for things to make any sense?

And finally, what bothers me is the ease with which ('ordinary') dark baryonic matter is disregarded. I concur with the general research that there would never be enough of it to explain all 'required' dark matter (although I wouldn't be totally surprised if this was based on an assumption no one realised that was erronous), but it can still be used to explain 'freaks' like the bullet cluster. So I guess I am in favour of more research there after all...

I have my hopes set on gravity wave detectors to give us some insight in this problem. A new type of eyes to watch the skies should help. And like everyone I will be delighted if we happen to find one or several exotic particles we could never have imagined, but at this stage we can not afford only one direction imo.

I like stories about topics like this one because it is very hard for the political zealots -- from the right OR the left -- to hijack the discussion with ideologically motivated, non-scientific palaver.

Neither theory is correct. Neither answer the question: how does gravity get out of a black hole? Until this is answered, all theories about gravity are moot. And yes, that included string theory.

I'm a total layperson, but isn't gravity the whole point of a black hole? That there's so much gravity that nothing else can get out?

It sounds like you're saying gravity should act on itself.

I'm not saying it. General relativity says that the gravity of a black hole is so intense that light does not travel fast enough to escape. If gravity travels at the speed of light (or slower), it too cannot escape. Therefore, black holes have no gravity. Or General Relativity is wrong. Or gravity is weirder than anyone thinks it is.

Gravity is just the distortion of space-time. Gravity doesn't need to "escape". Gravity isn't an object, gravity is an effect.

Many of the folks critical of dark matter are philosophically opposed to the idea of hypothesizing unseen or unmeasured phenomena to explain observed results that they cannot otherwise account for. And yet, this practice is far more common (and more successful) than a lot of people realize.

The problem with dark matter is not just that it is unseen, it is that it is unseeable.

It is no more unseeable than atoms where before the turn of the 20th Century. In fact NOBODY HAS EVER SEEN AN ATOM, yet we are quite certain that they exist, and Rutherford's experiments with gold foil were seen as definitive. However atoms were a well-accepted, and after the mid 19th Century virtually the only accepted, theory of matter. There were simply too many observations which collectively made sense in the context of atomic theory. As I say, even to this day, in a strict sense atoms cannot be seen, we can only see their effects on our instrumentation.

Dark matter is NO DIFFERENT from atoms. It explains a wide variety of phenomenon and it can be observed by its effects on other more visible things, galaxies and stars in this case.

Likewise broader hypotheses like the Big Bang are favored because, while they require as-yet-unexplained elements, they do fit together the vast majority of all observations into a coherent framework. I'd also like to note that at this time ALL our theories in physics rest on as-yet-unexplained elements since we have no fundamental explanations for mass, energy, space, or time or in fact any phenomena whatsoever. ALL of our theories are provisional and are ONLY justified by their explanatory power. If a "plasma universe" or "steady state" theory will provide equal or better explanatory power then I'm fairly sure that they will be accepted, though maybe grudgingly and over time, by the scientific community. Frankly my understanding of "plasma" cosmology is it is just totally unworkable at a basic level and based on flawed physically unrealistic math, but then I'm not a cosmologist so it is a bit hard for me to tell for sure. Steady State theories have their own problems, which I gather are at least as significant and similar to the objections raised to the Big Bang. Thus I think the problem is more that nobody has presented a model MORE APPEALING than the Big Bang, which does have a lot going for it.

Gravity is just the distortion of space-time. Gravity doesn't need to "escape". Gravity isn't an object, gravity is an effect.

You have it backward. Gravity causes space to curve. Gravity is a field, caused by mass. The mass must transfer the field through some means. The means must escape from a black hole or black holes will have no gravity.

Put me in the 'critical of dark matter' camp. Now, I only have my master degree in astrophysics (received in the university of the 'inventor' of dark matter no less...) and went on to new things after that, so I haven't got the most up-to-date understanding. However, I have never seen any observation that has really convinced me otherwise of my most basic observation of dark matter theory; That it in itself is a band-aid put there to explain observations that do not match our expectations using our current understanding of gravity.

I understand that it fits very well, but it fits very well like string theory fits very well. It is smart, it is tempting, but it is ultimately conceived to fix a hypothesis, not from observation. This is further strengthened by the fact that observations that do not actually prove the existence of dark matter, are used as proof. We already know that the models of gravity do not work without something to adjust, another observation proving this, does nothing to proof that the explanation is dark matter. As far as I know, any theory that produces falsifiable hypotheses has failed thus far or has yet to produce any evidence that supports it.Not unless we get a much firmer grip on what the dark matter might be will I ever be convinced I think.

A lot of the proof is also definitely circular in nature. We can not proof that dark matter has to exist by using gravitational models, since dark matter exists as an explanation for the discrepancy of our understanding of gravity and observations. I understand the tendency to use general relativity as 'fact', since any adjustment is inconceivable to most of us, but as scientists we must not fall into this trap.

Up until we find evidence of massive neutrino's, WIMPS and what not, I think it simply pays to remind ourselves of the 'weakness' of the dark matter theory. It is too easily accepted imho and I am happy at least some people are looking into alternatives.

I am not trying to be sarcastic with this response as I make the optimistic assumption that you actually do have a degree.

I remember reading semi-recently, because of computational power of modern computers, a group was able to crunch over the data of the observable sky-map. They were able to determine within a 99.9% certainty that based on red-shifting of light leaving the gravity well of remote galaxies, that gravity works identically at large scale as it does at local scale.

Assuming this is correct, then the non-existence of dark-matter would mean that our math is wrong when dealing with gravity, yet we know we have been dead-on.

To me, disproving dark-matter means disproving that we have satellites in orbit.

Again, this is the internet and assuming you have a degree, then I would respectfully love to hear your response.

That is not the same as "unambigous observation" of dark matter. Indeed, one of the major problems with dark matter is that it should be all around us (the Milky Way is rife with the stuff, at least according to proponents of dark matter), but if we look at the solar system it just doesn't seem to be there - the planets and the sun move according to the distrubution of visible matter. This is a rather large problem as, if it was present, we should be able to see its impact locally, and yet we don't.

It doesn't help that in many cases, we have to generate a "dark matter halo" whose distribution is suspect and, perhaps worse still, there are places where the "dark matter" and luminous matter don't line up at all, and there's no real reason WHY that would happen - gravity should affect both equally.

All this leads to the question of whether dark matter is real. After all, the purpose of a scientific theory is to make predictions, and right now, our theory does -not- predict things at all - we have to invent dark matter to make it "predict" galactic structure. That is always bad news - when you have to mold your observations to fit your theory, rather than vice-versa, you always have to ask yourself, repeatedly, if you are just deluding yourself.

Saying that it would appear that there is dark matter is FINE. But saying that is what actually is going on is incredibly suspect.

Actually this is perfectly well explained. Dark matter doesn't interact (much at least) with either itself or with normal baryonic matter. The result is that it cannot achieve thermal equilibrium. The velocity distribution of dark matter particles is entirely determined by their velocities at creation, they never cool down or heat up. This means their bulk behavior is profoundly different from that of ordinary matter. This fully accounts for the existence of halos of dark matter and its relative scarcity in the vicinity of the Solar System. Given that we don't know a lot about the behavior of dark matter in other respects there is no reason to suppose that we should observe any 'missing' phenomena. Presumably there MAY be a certain amount of fairly cold dark matter which is gravitationally bound to the Solar System, yes, but the quantity need not be high enough to have any detectable effect on celestial mechanics etc. In fact we may find ways to detect its subtle effects if it exists, but we're FAR from having ruled it out or even made it all that implausible at this time.

That is not the same as "unambigous observation" of dark matter. Indeed, one of the major problems with dark matter is that it should be all around us (the Milky Way is rife with the stuff, at least according to proponents of dark matter), but if we look at the solar system it just doesn't seem to be there - the planets and the sun move according to the distrubution of visible matter. This is a rather large problem as, if it was present, we should be able to see its impact locally, and yet we don't.

It doesn't help that in many cases, we have to generate a "dark matter halo" whose distribution is suspect and, perhaps worse still, there are places where the "dark matter" and luminous matter don't line up at all, and there's no real reason WHY that would happen - gravity should affect both equally.

All this leads to the question of whether dark matter is real. After all, the purpose of a scientific theory is to make predictions, and right now, our theory does -not- predict things at all - we have to invent dark matter to make it "predict" galactic structure. That is always bad news - when you have to mold your observations to fit your theory, rather than vice-versa, you always have to ask yourself, repeatedly, if you are just deluding yourself.

Saying that it would appear that there is dark matter is FINE. But saying that is what actually is going on is incredibly suspect.

Actually this is perfectly well explained. Dark matter doesn't interact (much at least) with either itself or with normal baryonic matter. The result is that it cannot achieve thermal equilibrium. The velocity distribution of dark matter particles is entirely determined by their velocities at creation, they never cool down or heat up. This means their bulk behavior is profoundly different from that of ordinary matter. This fully accounts for the existence of halos of dark matter and its relative scarcity in the vicinity of the Solar System. Given that we don't know a lot about the behavior of dark matter in other respects there is no reason to suppose that we should observe any 'missing' phenomena. Presumably there MAY be a certain amount of fairly cold dark matter which is gravitationally bound to the Solar System, yes, but the quantity need not be high enough to have any detectable effect on celestial mechanics etc. In fact we may find ways to detect its subtle effects if it exists, but we're FAR from having ruled it out or even made it all that implausible at this time.

Another explanation could be that a nearby supernova which may have helped form our solar system, blew all the dark matter away from this region of the galaxy.

Actually if your theory doesn't get everything right then that's a sign that your theory is wrong. Fact is Dark Matter is a theoretical construct they conjured up to try and fill in gaping holes in the current standard model....

I thought your post was wonderful until I googled Plasma Cosmology and Steady-state model.

I think the fact that they buried Plasma Cosmology multiple paragraphs into the letter was very telling. Kept me wondering if they were serious much longer than i would have otherwise.

Short summary: predicted solely because it made the math work, wasn't integrated into theory for years, didn't get detected for decades. There's a history here that those with "i don't like the feel of this" tendencies are largely ignoring.

While the example of the neutrino works well to demonstrate your point, It seems equally significant to address the exponentially larger number of such placeholders that ultimately didn't pan out. Statistically speaking, when discussing a random entity imagined without basis in observation for the exclusive purposes of propping up a popular theory, history shows that assertions made regarding such an entity are more likely to be false than true.

The ultimate acceptance of the neutrino does not appear able to impact the empirical accuracy of current theories related to Dark Matter one way or the other. It is simply a single data-point cherry-picked from a much larger set of information.

By proceeding forward under a belief despite lack of basis in direct observation, what institutional science does in this regard is eerily similar the approach of creationists - who are criticized heavily for it.

I think you missed my point which was, that the fact that this viewpoint worked here and didn't there, says nothing. The fact that we both agree on what is more or less the 'answer' now, is based on very different type of data, that verifies the actual hypotheses based on the observation that something was 'off'. Everything (including the filaments) we have currently, is just an extension of the same 'something is not right here' observation. We just keep verifying that something is indeed not right and that it is exactly not right as we thought it is not right (that was a bit of a weird sentence, but you get my meaning). It doesn't change our understanding of the underlying concept at all and is, imho of course, a waste of time and money.

I think you missed my point, Newtonian Mechanics is fine, all these observations that are consistent with GR are all just extensions of the same "something is not right here" observation about the precession of the perihelion of Mercury! The fact that we just keep verifying that something is indeed not right and that it is exactly not right as we thought it is not right. It doesn't change our understanding of the underlying concept at all and is, IMHO of course, a waste of time and money. Therefor we should all just go back to studying Newtonian Mechanics and forget about the GR thing that was just introduced to patch up some observations that didn't fit. Doing so is just a waste after all.

Clearly dark matter is not a 'waste' and is no more or less valid a hypothesis than many others. It isn't on the same level as GR yet, and is in fact not such a generalized theoretical construct, but it is absolutely a perfectly valid observed phenomenon and corresponding hypothesis. Maybe it will turn out instead that there's a 5th force or some form of MOND, or whatever, but so far delta CDM is the most successful model and is a perfectly fine testable hypothesis that is certainly not complete (at least until we know what CDM is), but is definitely not a waste to study.

Actually if your theory doesn't get everything right then that's a sign that your theory is wrong. Fact is Dark Matter is a theoretical construct they conjured up to try and fill in gaping holes in the current standard model....

I thought your post was wonderful until I googled Plasma Cosmology and Steady-state model.

I think the fact that they buried Plasma Cosmology multiple paragraphs into the letter was very telling. Kept me wondering if they were serious much longer than i would have otherwise.

Short summary: predicted solely because it made the math work, wasn't integrated into theory for years, didn't get detected for decades. There's a history here that those with "i don't like the feel of this" tendencies are largely ignoring.

While the example of the neutrino works well to demonstrate your point, It seems equally significant to address the exponentially larger number of such placeholders that ultimately didn't pan out. Statistically speaking, when discussing a random entity imagined without basis in observation for the exclusive purposes of propping up a popular theory, history shows that assertions made regarding such an entity are more likely to be false than true.

The ultimate acceptance of the neutrino does not appear able to impact the empirical accuracy of current theories related to Dark Matter one way or the other. It is simply a single data-point cherry-picked from a much larger set of information.

By proceeding forward under a belief despite lack of basis in direct observation, what institutional science does in this regard is eerily similar the approach of creationists - who are criticized heavily for it.

I'm not sure I agree with that. The neutrino for example was pretty well specified from the start, it was proposed to be a particle with quite specific properties (though some others were and even today are not entirely known). Most of the placeholders which people trot out which were not ultimately accepted or have been discarded are entirely different. Nobody ever nailed down ANY of the properties of 'phlogiston' nor of the 'luminiferous aether' etc. In fact these failed placeholders tend to have specific characteristics. They are not well-formed hypotheses, but are instead usually simple misplaced concepts that reflect a tendency to extend one form of reasoning to another domain when it is not justified. The aether for instance being a generalization of "there must be some medium for an effect to propagate through" and phlogiston simply being the assumption that everything real was material resulting in misapplied logic. Neither was ever AT ALL quantified and no observation was ever made which in any way supported the existence of either one. They were in fact unjustified and only 'conceptual crutches' not scientific hypotheses. When sufficiently rigorous hypotheses WERE formulated each of these ideas was instantly relegated and the replaced with more sophisticated concepts as our understanding grew (atomic theory lead to the development of statistical mechanics to explain heat and of course SR was developed to explain the behavior of light).

So, the question then in terms of a gut idea about DM would be to ask if it is simply a conceptual crutch or is it in fact simply a well-quantified hypothesis which explains facts by introducing a new element. I think the difficulty here is we're not in a good position to say. Our conceptual framework of the cosmos and of gravity seems to WORK, but we know there are still great gaps in our understanding which might revolutionize our insight and make DM a footnote in history. OTOH it is certainly more quantified than phlogiston ever was and more justified than the aether. Other candidates may overtake it, but at this point it seems foolish to relegate the hypothesis simply because we'd rather not introduce a new thing.

While the example of the neutrino works well to demonstrate your point, It seems equally significant to address the exponentially larger number of such placeholders that ultimately didn't pan out. Statistically speaking, when discussing a random entity imagined without basis in observation for the exclusive purposes of propping up a popular theory, history shows that assertions made regarding such an entity are more likely to be false than true.

I'm not sure, but i think we differ in our definition of placeholders. For me, a placeholder is something that becomes widely accepted and motivates further research. Not "oh, i thought of that briefly, but discarded it." I can only think of a handful of placeholders total. Given those numbers, it's just impossible to have an exponentially larger population of unsuccessful ones.

I think you missed my point which was, that the fact that this viewpoint worked here and didn't there, says nothing. The fact that we both agree on what is more or less the 'answer' now, is based on very different type of data, that verifies the actual hypotheses based on the observation that something was 'off'.

Based on this phrasing, i have no confidence that i understand what you're arguing at all. The same goes for the sentence where you assumed i would know what you're saying further down.

Kingmöb wrote:

Everything (including the filaments) we have currently, is just an extension of the same 'something is not right here' observation.

The entire Standard Model is a collection of extensions of "something is not right here" observations. So what? Once the evidence rolls in, it really doesn't matter what the original motivation for a model was. All that matters is "is this consistent with the data?"

Kingmöb wrote:

Doesn't it raise any alarms for you that we have to invent a mysterious dark energy and dark matter for things to make any sense?

I think it's fantastic. At least for dark matter, it makes a ton of fairly specific predications, which we are in the process of testing. However they come out, the results will be fascinating. Just getting to the point where we know what questions to be asking about dark energy is the same.

That is not the same as "unambigous observation" of dark matter. Indeed, one of the major problems with dark matter is that it should be all around us (the Milky Way is rife with the stuff, at least according to proponents of dark matter), but if we look at the solar system it just doesn't seem to be there - the planets and the sun move according to the distrubution of visible matter. This is a rather large problem as, if it was present, we should be able to see its impact locally, and yet we don't.

It doesn't help that in many cases, we have to generate a "dark matter halo" whose distribution is suspect and, perhaps worse still, there are places where the "dark matter" and luminous matter don't line up at all, and there's no real reason WHY that would happen - gravity should affect both equally.

All this leads to the question of whether dark matter is real. After all, the purpose of a scientific theory is to make predictions, and right now, our theory does -not- predict things at all - we have to invent dark matter to make it "predict" galactic structure. That is always bad news - when you have to mold your observations to fit your theory, rather than vice-versa, you always have to ask yourself, repeatedly, if you are just deluding yourself.

Saying that it would appear that there is dark matter is FINE. But saying that is what actually is going on is incredibly suspect.

Actually this is perfectly well explained. Dark matter doesn't interact (much at least) with either itself or with normal baryonic matter. The result is that it cannot achieve thermal equilibrium. The velocity distribution of dark matter particles is entirely determined by their velocities at creation, they never cool down or heat up. This means their bulk behavior is profoundly different from that of ordinary matter. This fully accounts for the existence of halos of dark matter and its relative scarcity in the vicinity of the Solar System. Given that we don't know a lot about the behavior of dark matter in other respects there is no reason to suppose that we should observe any 'missing' phenomena. Presumably there MAY be a certain amount of fairly cold dark matter which is gravitationally bound to the Solar System, yes, but the quantity need not be high enough to have any detectable effect on celestial mechanics etc. In fact we may find ways to detect its subtle effects if it exists, but we're FAR from having ruled it out or even made it all that implausible at this time.

Except as I understand it isn't. Every attempt to model dark matter's properties seems to fail. You can tweak the properties to make it work for a system, but you can't make it work for every system. You can give it an interaction profile so it will aggregate around a particular galaxy, but when you apply the same model to galaxies of different size or other smaller objects the models don't match observation.

So we don't really know what it is, don't know it's properties and can't seem to find any.

That is not the same as "unambigous observation" of dark matter. Indeed, one of the major problems with dark matter is that it should be all around us (the Milky Way is rife with the stuff, at least according to proponents of dark matter), but if we look at the solar system it just doesn't seem to be there - the planets and the sun move according to the distrubution of visible matter. This is a rather large problem as, if it was present, we should be able to see its impact locally, and yet we don't.

It doesn't help that in many cases, we have to generate a "dark matter halo" whose distribution is suspect and, perhaps worse still, there are places where the "dark matter" and luminous matter don't line up at all, and there's no real reason WHY that would happen - gravity should affect both equally.

All this leads to the question of whether dark matter is real. After all, the purpose of a scientific theory is to make predictions, and right now, our theory does -not- predict things at all - we have to invent dark matter to make it "predict" galactic structure. That is always bad news - when you have to mold your observations to fit your theory, rather than vice-versa, you always have to ask yourself, repeatedly, if you are just deluding yourself.

Saying that it would appear that there is dark matter is FINE. But saying that is what actually is going on is incredibly suspect.

Actually this is perfectly well explained. Dark matter doesn't interact (much at least) with either itself or with normal baryonic matter. The result is that it cannot achieve thermal equilibrium. The velocity distribution of dark matter particles is entirely determined by their velocities at creation, they never cool down or heat up. This means their bulk behavior is profoundly different from that of ordinary matter. This fully accounts for the existence of halos of dark matter and its relative scarcity in the vicinity of the Solar System. Given that we don't know a lot about the behavior of dark matter in other respects there is no reason to suppose that we should observe any 'missing' phenomena. Presumably there MAY be a certain amount of fairly cold dark matter which is gravitationally bound to the Solar System, yes, but the quantity need not be high enough to have any detectable effect on celestial mechanics etc. In fact we may find ways to detect its subtle effects if it exists, but we're FAR from having ruled it out or even made it all that implausible at this time.

Except as I understand it isn't. Every attempt to model dark matter's properties seems to fail. You can tweak the properties to make it work for a system, but you can't make it work for every system. You can give it an interaction profile so it will aggregate around a particular galaxy, but when you apply the same model to galaxies of different size or other smaller objects the models don't match observation.

So we don't really know what it is, don't know it's properties and can't seem to find any.

Here's the problem though: Look at all the vast array of galaxies out there. There are 100's of billions of them and no two look alike. Each one has a unique history and the exact distribution of luminous baryonic matter which we CAN see is thus different in each one. Now, would you expect the non-baryonic matter to be the same in each one? Of course not. We don't have ANY model that is entirely accurate for ANY aspect of galaxies (or groups/cluster/superclusters/filaments/etc) so why would you expect that you will magically create such a model for dark matter EVEN IF YOU KNEW ALL ITS PROPERTIES. These are HARD problems. They are even harder because we clearly have to distinguish between the effects of matter who's properties we do know from those we don't. Thus there are a whole host of unknowns, and when we apply our models for an individual galaxy all we can do is assume some default generic history. In reality we cannot say how that unique galaxy or whatnot DID evolve. Without knowing that ALL we can do are statistical analyses of the whole population, we can never fully explain any one example (though we might eventually understand the parameter space well enough to constrain them to a few basic cases).

Except as I understand it isn't. Every attempt to model dark matter's properties seems to fail. You can tweak the properties to make it work for a system, but you can't make it work for every system. You can give it an interaction profile so it will aggregate around a particular galaxy, but when you apply the same model to galaxies of different size or other smaller objects the models don't match observation.

So we don't really know what it is, don't know it's properties and can't seem to find any.

I'm not sure what you mean by "interaction profile". Dark matter (is postulated to) interact with other dark matter, and everything else, via gravity. It has the property that it interacts via gravity and nothing else - I'm not sure what tweaking of properties you are referring to.

Many of the folks critical of dark matter are philosophically opposed to the idea of hypothesizing unseen or unmeasured phenomena to explain observed results that they cannot otherwise account for. And yet, this practice is far more common (and more successful) than a lot of people realize.

Back in the late 1700s, after the discovery of the planet Uranus, astronomers made careful calculations of its orbital elements and published a table of the positions of the planet in the sky over the years (and decades). As the years (and decades) wore on, they discovered a curious thing: the actual position of the planet was beginning to diverge from what had been predicted.

At this point, there were a few different explanations:

1) Perhaps the initial orbital elements were incorrect.2) Perhaps our fundamental laws of gravity and motion were incorrect.3) Perhaps there was a massive, as-yet-undetected eighth planet whose gravity was influencing the orbit of Uranus.

Most astronomers fell into the third camp; after all, the observations of Uranus's orbit had been made with considerable precision (for the time) and there was little reason to believe that the fundamental laws of physics would start to break down as you move further away from the sun. And so they made their calculations and narrowed down the location of this hypothetical planet to a fairly small window in the sky. After that, it was just a matter of pointing a telescope there and looking.

This is the story of the discovery of the planet Neptune.

Astronomers did not find this planet by accident. It was not discovered by a kid in the backyard with a streak of cosmic good luck. (In fact, many observers from antiquity had seen it, but had not realized what they were looking at.) They found it because they knew it had to be there.

Now, you might think that this comparison is a bit of a stretch. But it's just one example; there are countless more. Back in 1930, Wolfgang Pauli was studying beta decay in atomic nuclei. He realized that the process, as he was seeing it, could not possibly be happening unless there were (again, hypothetical) particles being emitted as a consequence. If there were not, then all sorts of fundamental principles of physics were being violated (e.g., conservation of matter / angular momentum / etc.)

This particle, eventually named the "neutrino", remained hypothetical and undetected for more than a quarter of a century until it was finally detected -- in 1956.

I could go on, but the point is that postulating the existence of something hypothetical in order to explain deviations between theory and observed results is part of the best traditions of natural science. It's not hand-waving or charlatanism. And it works more often than most people might think.

(Note: This is largely copied-and-pasted from a post that I made on Slashdot about a year ago, but it fits in well with this topic.)

Pretty good but the claim about Neptune being observed from antiquity is mistaken. Its not visible to the naked eye. Galileo made observations of it but apparently mistook it for a fixed star. You also could have used the most obvious example of the hypothetical to observed, which of course, would be the atom.

@AlhazredTrue, but of course that is why it is still, to my mind, an open question. I don't mean that in a "who knows" kind of way (well maybe a little). But more in a, every model so far seems to have strongly contradictory features. In order to get one thing to work, another pops out.

I have to admit probably being more biased against dark matter than your average bear. Still, each side is better at pointing out the flaws in the other side than giving evidence making their belief concrete.

It doesn’t feel like just need to find the “right model” but more of a we need an “ah ha” moment. We are doing a bad job of showing how speculative all this is, I get questions from friends about what DM is and I have to explain just how little we know and how inconsistent the evidence is.

Except as I understand it isn't. Every attempt to model dark matter's properties seems to fail. You can tweak the properties to make it work for a system, but you can't make it work for every system. You can give it an interaction profile so it will aggregate around a particular galaxy, but when you apply the same model to galaxies of different size or other smaller objects the models don't match observation.

So we don't really know what it is, don't know it's properties and can't seem to find any.

I'm not sure what you mean by "interaction profile". Dark matter (is postulated to) interact with other dark matter, and everything else, via gravity. It has the property that it interacts via gravity and nothing else - I'm not sure what tweaking of properties you are referring to.

Well... remember it has to form halo's around objects of a certain size, but not objects of another, normally smaller size. So it needs to be able to "cool down" and be captured in some cases, but not "cool down" and be captured in a system of a smaller scale.

This would be much less bad if we could show some model to predict the halo of most galaxies, but we can't and the requirements so far have appeared contradictory.

@AlhazredTrue, but of course that is why it is still, to my mind, an open question. I don't mean that in a "who knows" kind of way (well maybe a little). But more in a, every model so far seems to have strongly contradictory features. In order to get one thing to work, another pops out.

I have to admit probably being more biased against dark matter than your average bear. Still, each side is better at pointing out the flaws in the other side than giving evidence making their belief concrete.

It doesn’t feel like just need to find the “right model” but more of a we need an “ah ha” moment. We are doing a bad job of showing how speculative all this is, I get questions from friends about what DM is and I have to explain just how little we know and how inconsistent the evidence is.

I understand the feeling. Dark Matter does seem to WORK rather well though. I think it could easily be that there are a couple of different types of dark matter, or it could have some unusual properties we simply have not had any reason to guess exist, etc. In other words there seem to still be some unknowns, but the basic CDM concept fits well, and it is reasonably logical. We've ruled out baryonic matter, and we've ruled out any of the known stable particles. MOND doesn't seem to work at all on a large scale, and even at best the versions that are being worked on now are VERY ugly and look extremely suspiciously like curve fitting exercises. So what are we really left with? Either a very inelegant math that will get more inelegant before it fits the large scale, at best, and from which you can't as far as we know produce a version of GR. Option 2 is dark matter, and option 3 is "voodoo", something utterly new that hasn't even been suggested yet.

Dark matter might not be perfect and it might still feel a bit like an ad-hoc solution, but it has promise that from what I can see the alternatives, such as they are, lack. The likelihood is too that to get some other model to work you will be stuck cosmologically, which means for instance MOND certainly actually leaves you WORSE off, even if it can explain everything we observe it will do it only at the expense of our entire cosmology, which then opens huge additional questions even bigger than what was answered.

Except as I understand it isn't. Every attempt to model dark matter's properties seems to fail. You can tweak the properties to make it work for a system, but you can't make it work for every system. You can give it an interaction profile so it will aggregate around a particular galaxy, but when you apply the same model to galaxies of different size or other smaller objects the models don't match observation.

So we don't really know what it is, don't know it's properties and can't seem to find any.

I'm not sure what you mean by "interaction profile". Dark matter (is postulated to) interact with other dark matter, and everything else, via gravity. It has the property that it interacts via gravity and nothing else - I'm not sure what tweaking of properties you are referring to.

Well... remember it has to form halo's around objects of a certain size, but not objects of another, normally smaller size. So it needs to be able to "cool down" and be captured in some cases, but not "cool down" and be captured in a system of a smaller scale.

This would be much less bad if we could show some model to predict the halo of most galaxies, but we can't and the requirements so far have appeared contradictory.

Gravitational interactions alone allow for this sort of thing. There is a range of velocities in DM particles. The faster ones fly off, the slower ones stick around. Depending on the spectrum of velocities it is quite possible for DM to exist in the galaxy but not be trapped by the Sun to any great degree.

To go out on a limb a bit, there will be no such thing Dark Matter; what they will find, is that normal matter can "over displace" space-time, and manifest more gravity than it should..

How will that account for gravitational lensing by apparently empty space?

Great. A staff member calls me out and of course I'm working a sixteen hour day.

Well the easy answer would be mass between us and the light source. Almost a trick question eh? I just don't see why it has to be "dark" matter, or dark energy for that matter.

But actually, thinking about those labels; certainly dark energy could be used as a term to describe what I proposed above. That normal matter can over-gravitate under the duress of a black hole. (How that happens, is I think, explored in string theory; one could think of it as too much mass has overloaded space-time, and "punched through" to another dimension.) If that is true, then the gravitational constant is off the table (or at least the table just got a lot bigger). Once you begin to question g, then the squares of their distances are also no longer givens.

So, given the 84% of the universe estimate (just for matter, not even energy), would that mean an average of about 6 times "normal" gravity for the entire universe? That would seem to imply an average black hole would be generating a gravitational field strength of hundreds or thousands of times normal. But not just in strength; which would have been easily observed at the centers of galaxies by now I imagine; but distance as well.

Perhaps dark energy is a good label after all for this (theoretical) phenomena. And dark matter is what I may call the increased observed gravity from this dark energy spilling into our universe.

Sort of, but these aren't photos taken with light. We can't actually see the electron beams or super fine atomic force probes used to make these images. How do we know these molecules and atoms are REAL and not just empty explanations for the measurements we record? Nobody has SEEN an atom. They are in fact in a very deep sense hypothetical entities which are incomprehensibly small and numerous. More atoms exist in the head of a pin than all the human beings who will ever exist in all of time. How real are these things to us? How much less real really is dark matter than that? In each case something we cannot sense or directly understand or experience exists only in relation to the indirect effects that it presents to our senses.

I'm not saying that atoms don't exist of course, but every argument along the lines of "it isn't real, it is just made up to explain other things", EVERY SINGLE SUCH ARGUMENT must also cause us to dismiss the existence of atoms. You cannot hold a consistent viewpoint and reject one on those grounds and yet refuse to reject the other. Again, that doesn't mean we have to accept dark matter, just that IF it proves to be a sufficient explanation then we cannot reject it on grounds of 'unreality' and not reject most of 20th Century physics. Heck, we can't even accept the existence of electromagnetic and gravitational fields by that logic. This way lies madness.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.